Cooling & Heating Fundamentals

This column is the fifth in a series exploring refrigeration and heat pump concepts without using jargon.

Some people are more passionate about leakage reduc­tion than others.

System designers go to great lengths to ensure their equip­ment is capable of dealing with the sorts of variation in load that were described in the June column. This includes the changes in the amount of working fluid that is contained in the cooling part of the circuit. If the cooling requirement is large then lots of liquid will be boiling and most of the space in the cooler will be taken up with gas. However, when the load reduces and the boiling is less vigorous then more of the space is filled with liquid, which has to come from somewhere else in the system. This is why systems need a tank with a lot of liquid in it, so that the plant doesn’t fail to perform when the load is light and more liquid is required in the cooler.

This leads to two specific dif­ficulties for the operator of the system. First, it is very difficult to say what is the right liquid level in the tank because that depends on what is happening in the rest of the system. Second, if some of the working fluid leaks out of the system its disappearance might not be immediately obvious. However, just at the time it is most needed it will not be there, and the system is likely to misfire in one way or another. A shortage of working fluid might cause the system to run less efficiently than it should or to stop working altogether.

Leakage of working fluid causes many other problems. The fluid might be toxic, or flammable, or smelly or perhaps harmful to the environment in some less obvious way. Even if it is non-toxic it might suffocate people who are working near the leak or who walk into a room in which the oxygen has been displaced by colorless, odorless refrigerant. If a system is “short of gas” then someone has to purchase some more, someone has to get in their truck and drive to the site and someone has to return the empty cylinders once the job is done. However, if the leak isn’t found and fixed then they will be back next month, and the month after, and the month after that. In some countries “topping up” a system that is known to be leaking is illegal and carries a heavy fine if convicted.

Leaks are therefore bad news and should not be allowed to happen. The good news is that most of them can be prevented, with a bit of care, some forward planning and a dose of common sense. Most leaks come from broken pipes, worn or faulty seals and loose fittings. Pipes can be broken by the effects of exces­sive vibration, either through fatigue failure, through abrasion of the pipe against another object or through work-hardening. Pipes also fail due to corrosion, particularly under insulation if the vapor seal is not maintained. Many of these causes can be eliminated at the design stage of the system by using suit­able materials, ensuring adequate clearances around pipes and avoiding screwed fittings wherever possible.